In the Latin Vulgate Bible, Saint Jerome renders the Greek word epiousion in the fourth petition of the Our Father both as “daily” (quotidianum) and as “supersubstantial” (supersubstantialem),

The Our Father is recorded in the Gospels of Matthew and Luke. In Matthew, Saint Jerome uses the Latin words for supersubstantial bread whereas in Luke he uses the Latin words for daily bread.

I have always wondered why.

We are all, as it were, beggars when we petition God for what is necessary. Our Father knows that we have need of food and He supplies it. In fact, just prior to giving us the prayer to the Father, Jesus counsels us not to worry about what we shall eat or wear, for God already knows we have these needs. What, therefore, do we ask for in this petition for bread? We ask to receive something greater than bread and that is God Himself. We groan as suppliants. And what do we ask from God but Christ who tells us: “I am the living Bread which came down from heaven” (John 6:41)?

There are early commentators who translate the Greek text ton arton hemon ton epiousion as “sufficient bread” or “tomorrow’s bread” but they do not go chronologically beyond that, for we must be content with today and tomorrow and leave the future to God’s paternal providence. Such was the manna in the desert; it served the Hebrews daily and on Friday for the morrow, that is for the last day of the week, the sabbath. And Jesus in His sermon on the Eucharist in John VI refers to His Body and Blood as the “living bread” that has come down from heaven, giving eternal life, not as the manna, which had no power to confer immortality.

It seems to me that Saint Jerome, in translating the inspired Greek into Latin, was paying respect (as Cornelius a Lapide notes) to the Gospel Manuscript versions of the Syriac, Arabic, Egyptian, Persian and other codices, which rendered epiousion as “daily” or “essential.” Even though Saint Jerome accepts other translations for the word (he himself employed words such as “principal,” “glorious,” and “excellent,” Cornelius a Lapide affirms that the great doctor gives epiousion a Eucharistic interpretation: “Jerome’s reason for translating ε̉πιούσιον literally by supersubstantial, was to indicate that in this petition we ask, above all, for heavenly bread, such as we receive in the Eucharist.” So, too, do Cassian (Collat. 9. 20), Saint Cyril (Cat. Mystag. 5), and Saint Ambrose (lib. 5, de Sacrament. c. 4), the latter of whom, says a Lapide, “by this bread understands the Eucharist which in Zech. ix. is called ‘the corn of the elect.’”

Our trusty exegete, Cornelius a Lapide, concludes his commentary with this summary of the fourth petition of the divine prayer::

“[T]hat this bread is both material, for the sustenance of the body, and spiritual and heavenly bread, suitable for the nourishment of the soul, such as the word of God and the Eucharist. We have need of both, and therefore we ought to ask for both, and for the latter so much more earnestly than the former, as the soul is superior to the body. And this is denoted by the word supersubstantial, which S. Jerome explains to mean superexcellent, surpassing all created substances, because, as Cassian says, ‘the sublimity of its magnificence and its sanctity is superior to that of the whole creation.’ And for this reason, in the Greek, the definite article is added, doubled in truth, τὸν άρτον τὸν ε̉πιούσιον, ‘the bread the supersubstantial.’ As though it were said, ‘Give us bread not common, but celestial and divine.’ Christ alludes to the manna given to the Hebrews, which was a type of the Eucharist. For of manna, it is said in Ps. lxxviii. 24, ‘He gave them bread from heaven.’ ‘Man did eat angels’ food.’ Thus, therefore, manna was food ε̉πιούσιος, i.e., heavenly and angelic; but much more is the Eucharist. . . . S. Ambrose calls the Eucharist this supersubstantial bread. ‘If,’ he says, ‘this be daily bread, why do you receive it only once a year? So live that you may be fit to receive it daily.’ Thus the first Christians were accustomed to communicate daily, as is plain from Acts ii. 46. And S. Cyprian (de Orat. Domin.) says, ‘We ask that this bread may be daily given us, lest we, who are in Christ and daily receive the food of the Eucharist, by the intervention of some grave fault, by abstaining and not communicating, should be kept back from the heavenly Bread, and separated from the Body of Christ, when He Himself has admonished us saying, ‘I am the Bread of life, Who came down from Heaven. If any man shall eat of My Bread he shall live for ever.’” (S. John vi.)